A vicious type of Saber-toothed hunchback that inhabits the Ural Mountains. While this is its native habitat Aggys have been known to live outside of Europe even in Australia. One such Aggy was discovered by a famous ex-KGB spy and master of disguise Ryksvell Fyttsgerrg while using the alias Guy Burnside (whereupon he did remark in his video diary "very strange to see Aggy outside Russia will study habits of such an anomily but in meantime continue pretending to be Australian child). As is the cunning nature of an Aggy it had worn down its sabre-teeth and used the pseudonym 'eggy' to diguise it's true nature and fit into a human society. The Aggy is well know throughout eastern Europe for its ferocity and habit of living in caves and being covered in a thin layer of fur. If confronted by an Aggy under no circumstances fight it unprepared as the only way to kill one is with a chiropractor through the heart.

Ryksvell Fyttsgerrg: my god a real Aggy, in Australia?
Eggy (clever hunch disguise for Aggy): wft are you talking about you retard
Ryksvell Fyttsgerrg: very clever Aggy, i'll get you one day

the word derives from the adjectives 'aggressive' and 'agitated' and is the emotion felt when seriously angry and annoyed. Signs of 'agg' can often be seen in the eyes and mouth.

The best way of preventing a friend from getting aggy is to say "Mate, don't get aggy!"

Dave had just lost his fifth consecutive game of FIFA and was clearly getting aggy.
On closer inspection the pupils in his eyes were dilating and turning red, whilst his mouth was shaking uncontrollably with rage. He was so aggy.

Aggy can mean anything, although it's usually bad. It was originally used to described something annoying, because it came from the word aggrivating, but now it has evolved into a a word that can be used as pretty much any adjective, good or bad.